Process for the preparation of polymer solutions

ABSTRACT

A POCESS FOR MAKING SOLUTIONS IN ORGANIC SOLVENT OF ACRYLIC POLYMERS BY DISPERSION POLYMERIZING MONOMER IN AN ORGANIC LIQUID IN WHICH IT IS SOLUBLE, BUT IN WHICH THE RESULTING POLYMER IS INSOLUBLE AND FORMS DISPERSE POLYMER PARTICLES, THE REACTION BEING CARRIED OUT IN THE PRESENCE OF A STABILIZER HAVING A CONSTITUENT WHICH BECOMES ASSOCIATED WITH THE DISPERSE POLYMER PARTICLES AND A PENDENT CHAIN-LIKE CONSTITUENT WHICH IS SOLVATED BY THE ORGANIC LIQUID AND PROVIDES A STABILIZING SHEATH AROUND THE POLYMER PARTICLES, THE DISPERSE POLYMER PARTICLES THEN BEING SEPARATED FROM THE MAJOR PART OF THE ORGANIC LIQUID OF THE DISPERSION AND DISSOLVED IN ORGAINC SOLVENT. PREFERABLY, THE POLYMER IS TRANSFERRED DIRECTLY INTO SOLUTION MAKING THE DISPERSION IN A RELATIVELY LOW BOILING NONSOLVENT LIQUID AND THEN ADDING A HIGHER BOILING NONTO THE DISPERSION AND EVAPORATING THE LOWER BOILING LIQUID. PREFERABLY THE DISPERSE POLYMER PARTICLES PRODUCED BY THE POLYMERIZATION HAVE A PARTICLE SIZE LARGER THAN IS THE CASE WHERE THE DISPERSIONS ARE USED AS SUCH IN COATING COMPOSITIONS; A PARTICLE SIZE RANGING FROM 0.5 TO 5.0 MICRONS IS SUITABLE.

United States Patent 3,808,169 PROCESS FOR THE PREPARATION OF POLYMERSOLUTIONS Desmond Wilfrid John Osmond, 22 Clewer Park Windsor,Berkshire, Iver Heath, England, and Morice William Thompson, Urishay,Highfield Lane, Cox Green, Maidenhead, Berkshire, England No Drawing.Continuation of application Ser. No. 436,305, Mar. 1, 1965. Thisapplication Mar. 10, 1972, Ser. No. 233,731 Claims priority, applicationGreat Britain, Mar. 13, 1964, 10,728/ 64 Int. Cl. C08f 45/26, 47/20 US.Cl. 26034.2 8 Claims ABSTRACT OF THE DISCLOSURE A process for makingsolutions in organic solvent of acrylic polymers by dispersionpolymerizing monomer in an organic liquid in which it is soluble, but inwhich the resulting polymer is insoluble and forms disperse polymerparticles, the reaction being carried out in the presence of astabilizer having a constituent which becomes associated with thedisperse polymer particles and a pendent chain-like constituent which issolvated by the organic liquid and provides a stabilizing sheath aroundthe polymer particles, the disperse polymer particles then beingseparated from the major part of the organic liquid of the dispersionand dissolved in organic solvent. Preferably, the polymer is transferreddirectly into solution by making the dispersion in a relatively lowboiling nonsolvent liquid and then adding a higher boiling solvent tothe dispersion and evaporating the lower boiling liquid. Preferably, thedisperse polymer particles produced by the polymerization have aparticle size larger than is the case where the dispersions are used assuch in coating compositions; a particle size ranging from 0.5 to 5.0microns is suitable.

This is a continuation of prior US. application Ser. No. 436,305, filedMar. 1, 1965, and now abandoned.

This invention relates to processes of making solutions of acrylicpolymers particularly suitable for use in coating compositions and topolymer solutions thereby produced.

It is well known that when used as film-forming materials in coatingcompositions acrylic polymers should be of high molecular weight becauseof the improvement in toughness and resistance of the film formed fromsuch polymers. It is also Well known that as the molecular weight isincreased, so is the viscosity of a solution of the polymer; but sincethere is a practical limit to the viscosity of solutions used in coatingcompositions, either more powerful and usually more expensive solventsmust be used for higher molecular weight polymers or a reduction insolids content of the solution must be accepted.

Much of the difiiculty in dealing with solutions of acrylic polymers ofmolecular weight over 50,000 is due to the fact that the polymers madeby normal solution or aqueous pearl polymerization processes are of aWidespread range of molecular weights and usually contain a smallproportion of molecular weight 500,000 or more. Although the molecularweight of a polymer may be expressed as, say, 100,000, this is anaverage figure and the polymer is, in fact, a mixture of polymers ofmolecular weights which may range from 30,000 to 300,000. This 3,808,169Patented Apr. 30, 1974 ice widespread range of molecular weights givesrise to two disadvantages. Firstly, the lower molecular weight polymercontent had an adverse effect on such film properties as toughness andresistance. Secondly, the higher molecular weight polymer content exertsan adverse effect on the application properties of the solution which isout of proportion to any beneficial influence it might have on filmproperties.

We have found that acrylic polymers made by dispersion polymerizationprocesses have much better characteristics when applied from solutionthan similar polymers of the same average molecular weight produced bysolution or aqueous pearl polymerization.

By dispersion polymerization we mean a process of polymerizing acrylicmonomer in an organic liquid in which the monomer is soluble and theresulting polymer is insoluble and forms disperse particles, the liquidcontaining a stabilizer for the disperse particles of polymer producedby the polymerization which provides a stabilizing sheath around thepolymer particles.

Such dispersion-polymerized polymers are advantageously used in solutioncoating compositions.

The present invention provides, therefore, a process for makingsolutions in organic solvent of acrylic polymers by dispersionpolymerizing monomer in an organic liquid in which it is soluble, but inwhich the resulting polymer is insoluble and forms disperse polymerparticles, the reaction being carried out in the presence of astabilizer having a constituent which becomes associated with thisdisperse polymer particles and a pendent chain-like constituent which issolvated by the organic liquid and provides a stabilizing sheath aroundthe polymer particles, the disperse polymer particles then beingseparated from the major part of the organic liquid of the dispersionand dissolved in organic solvent.

By acrylic polymer is meant a polymer of an ester or amide of acrylic ormethacrylic acid or a copolymer of such an ester with anothercopolymerizable monomer. Suitable esters include those of alcoholscontaining 1-8 carbon atoms, such as methyl, ethyl, butyl and 2-ethoxyethyl methacrylate and ethyl acrylate. Suitable amides includeacrylamide, methacrylamide,tetriary butyl acrylamide and primary alkylacrylamides. Mixtures of such esters or amides may be copolymerized orone or more of the esters may be copolymerized with a higher alkyl esteror amide or acrylic or methacrylic acid or with another monomercontaining a copolymerizable vinyl group, e.g. itaconate esters,maleates and allyl compounds.

Not all the organic liquid of the dispersion need be separated but it ispreferred to remove as much as possible since the lower the residualcontent of the non-solvent from the dispersion less solvent or lesspowerful solvent is needed to dissolve the polymer. The disperseparticles of polymer may be separated from the organic liquid of thedispersion by coagulation, centrifugation, filtration, spraydrying,fluidized-bed drying, or other means, before being dissolved in organicsolvent.

Preferably, however, the polymer is transferred directly into solutionby making the dispersion in a relatively low boiling non-solvent liquidand then adding a higher boiling solvent to the dispersion andevaporating the lower boiling liquid. The addition of solvent andevaporation of non-solvent may take place consecutively or concurrently.A particularly advantageous embodiment of this direct transfer method isone in which the dispersion is heated to about its refluxingtemperature, the solvent is heated to at least as high a temperature andis added to the dispersion and non-solvent is then separated bydistillation. Boiling points of the liquids may range from 40 C. to 150C. but preferably liquid used in the dispersion boils in the range 6090C. in which case the solvent used to dissolve the polymer by the directtransfer method has a boiling point above 100 C.

The organic liquid of the dispersion is a non-solvent for the polymer;aliphatic hydrocarbons, optionially with a minor proportion of aromatichydrocarbon, are very suitable. The most suitable solvents will varyaccording to the precise nature of the polymer to be dissolved but inmany cases this invention makes possible the use of large proportions ofthe cheaper solvents such as toluene and acetone. Even when usingsolvents of this type it is pos sible, according to this invention, toformulate coating compositions which at spraying viscosity contain from20%30% by weight of polymer. Similar coating compositions based onpolymer made by conventional solution processes contain only 12-14%polymer at this viscosity.

Preferably, the disperse polymer particles produced by thepolymerization have a particle size larger than is the case where thedispersions are used as such in coating compositions; a particle sizeranging from 0.5 to 5.0 microns is suitable. This facilitates transferof the particles into solution, particularly where direct transfer isinvolved, since the rate of swelling of the particles by the solventduring the transition from dispersion to solution is more controllable.When the polymer is separated from the organic liquid of the dispersionbefore being dissolved, the larger size particles are also more easy toseparate and dry.

Particle sizes in the preferred range may be achieved using stabilizeror stabilizer precursor as hereinafter explained in a proportion of from0.1 to 1% by weight of the disperse polymer.

Dispersion of the larger size polymer particles may be made by oneembodiment of the process of this invention in which, during thecontinuous addition of the monomer, stabilizing block or graft copolymeror a precursor thereof is also added with the monomer, this concurrentaddition being discontinued before addition of the monomer is completed.Conveniently the concurrent addition of stabilizer or precursor isdiscontinued after addition of from 40% to 70% of the monomer.

In a further embodiment the organic liquid in which the polymerizationis carried out contains a minor proportion of a solvent for the polymerthough, of course, the mixture as a whole must be a non-solvent for thepolymer, by which we mean that at least 95% of the polymer is insolubletherein. The solvent may be of the type used to form the final solutionof polymer. This minor proportion of solvent which, according to thenature of the solvent may be up to 40% by weight of the liquid, assistsin coarsening the particle size of the dispersion, and in the case wherecopolymers are being formed and dispersed, it may assist in dissolvingone or more of the co-monomers and improve the stability of thedispersed particles of copolymer.

The major proportion of the monomer, which includes mixtures of monomerswhere co-monomers are used, is preferably added during the course of thereaction, this major proportion usualy being not less than 80%. Further,the rate of addition, which preferably is continuous, is such that theproportion of free monomer dissolved in the reaction liquid during theaddition is not more than 50% by weight of the reaction liquid.

Where the polymers are to be used in coating compositions, the molecularweight is less than 250,000 and so preferably a chain transfer agent isused in the polymerization reaction, this agent being of a polaritysimilar to that of the monomer so that it is distributed between theorganic liquid and polymer particles in a proportion similar to that ofthe monomer. Further, it shoud have a chain transfer co-efiicient ofapproximately unity in relation to the monomer. For example, inpolymerizations of acrylic monomer carried out in non-polar liquids suchas aliphatic hydrocarbons, alkyl mercaptans wherein the alkyl groupcontains from 4-10 carbon atoms are preferred.

If the polymer is appreciably soluble in the monomer the reaction shouldbe carried out under reflux conditions, the monomer and correspondingquantities of stabilizer, initiator and chain transfer agent being addedto the returning reflux stream.

The initiator used will depend on the nature of the polymerizationprocess. For example, as stated above, the polymer particles arestabilized as they are formed and in one embodiment of the inventionthis stabilizer is a block or graft copolymer having one polymericconstituent which is solvated by the organic liquid and anotherpolymeric constituent of different polarity which associates with thedisperse polymer. This copolymer may be formed simultaneously with thedisperse polymer by having present in solution in the reaction liquid apolymer (termed precursor) which can be copolymerized with a minor partof the monomer to form the necessary block or graft copolymer. Typicalexamples of these soluble, and therefore solvated, polymeric precursorswhich form the solvated polymeric constituent of the stabilizing blockor graft copolymer are:

polymers of long chain esters of acrylic or methacrylic acid, e.g.stearyl, lauryl, octyl, 2-ethyl hexyl and hexyl esters of acrylic ormethacrylic acid; polymeric vinyl esters of long chain acids, e.g. vinylstearate;

polymeric vinyl alkyl ethers; polymers of ethylene, propylene, butadieneand isoprene; and long chain fatty acids and polymers of hydroxy fattyacids.

It is preferred that the copolymerization takes place through anunsaturated group present in the precursor which group copolymerizes invinyl-type manner With the main monomer. Such precursors may be made,for example, as described in our U.S. Pat. No. 3,317,635, by re acting acompound containing both a C=C group and a reactive group with anaddition polymer containing complementary groups which will react by acondensation reaction with the compound. The resulting linkage may be anester, ether, amide or urethane link. For example, an addition polymercontaining carboxyl groups may be reacted with C=C unsaturated compoundcontaining glycidyl groups to form an ester link between the polymer andthe compound containing the C=C group. In this case both the mainpolymerization and the minor copolymerization are vinyl-type and theless energetic initiators, such as azodiisobutyronitrile, may be used.However, where such unsaturated groups are absent from the precursor andthe stabilizing copolymer is formed by side reaction grafting of monomeronto the precursor, more powerful initiators, such as peroxides, may berequired.

When the stabilizing block or graft copolymer is produced in situ from asoluble precursor, the polymeric constituent grafted onto the precursoris produced from the same monomer or mixture of monomers as the maindisperse polymer and so is inherently of the same general nature as themain disperse polymer; at the same time, because it is of the samegeneral nature as the insoluble disperse polymer it is of differentpolarity to the solvated polymeric constituent of the copolymer. As aresult, this polymeric constituent of the stabilizing copolymer readilybecomes associated with the similar polymer in the disperse particles asthey are formed and in this way the stabilizing solvated polymericconstituent of the copolymer becomes attached to the disperse particles.In general, Where the main disperse polymer is polar, the secondpolymeric constituent of the block or graft copolymer Will also be polarand the solvated polymeric constituent will be non-polar.

We have found that acrylic polymers made by these dispersionpolymerization processes have better solution characteristics thanpolymers made by solution processes, and that solutions ofdispersion-polymerized polymer have much better application propertieswhen used in coating compositions. The possibility of using mildsolvents such as toluene is an advantage in formulating re-finishcoating compositions since the milder solvents reduce the risk of there-finish causing crazing of the underlying old coating.

An alternative advantage is that polymers of higher molecular weight maybe used in the same solvents without giving rise to application problemssuch as cobwebbing.

A further advantage of this invention is that when solutions ofcopolymers are prepared, the copolymers are of more uniform compositionthan those prepared by solution polymerization.

When used in coating compositions, the solutions of polymers areplasticized and pigmented in the usual manner.

The invention is illustrated by the following examples in which partsare by weight unless otherwise stated:

EXAMPLE 1 A copolymer of 98 parts of methyl methacrylate and 2 parts ofmethacrylic acid was made by dispersion-polymerization in 7090 C.boiling point petroleum ether using a graft copolymer of polymethylmethacrylate and polylauryl methacrylate as the stabilizer.

Initial charge Parts 30% solution of graft copolymer polymethylmethacrylate/polylauryl methacrylate in petroleum ether (B.P. 7090 C.)3.4

Petroleum ether (B.P. 7090 C.) 320.0 solution of p-octyl mercaptan inpetroleum ether (B.P. 7090 *C.) 5.0 Azodiisobutyronitrile 1.5 Methylmethacrylate 32.0 Methacrylic acid 0.6

The mixture was refluxed for minutes, after which time it had turnedwhite. A mixture of:

Parts Methyl methacrylate 239.0 Methacrylic acid 4.9Azodiisobutyronitrile 0.5 p-Octyl mercaptan solution (as above) 8.8Graft copolymer solution (as above) 23.0

was then added to the returning reflux stream over a period of 1% hours.

A mixture of:

Parts Methyl methacrylate 239.0 Methacrylic acid 4.9Azodiisobutyronitrile 0.75 p-Octyl mercaptan solution (as above) 8.8

was then added to the returning reflux stream over a period of 1% hoursand reflux was continued for a further /2 hour.

The reflux stream was then diverted so that petroleum ether was removedby distillation, toluene being added to the reaction product at twicethe rate at which the petroleum ether was removed. When substantiallyall the petroleum ether had been removed there was left a 35% solutionof methacrylate copolymer in toluene.

At solids, the solution of copolymer in toluene had a viscosity of1.2-1.5 centipoises at 25 C. The copolymer, when dissolved in a mixtureof 95 parts by volume of ethylene dichloride and 5 parts by volume ofethyl alcohol at 0.5% solids at 25 C., had a reduced viscosity of 0.4which is equivalent to a viscosity average molecular weight of100,000120,000.

The solids solution of the polymer in toluene was plasticized withdicyclohexyl phthalate at 30 parts of plasticizer to 70 parts of polymerand adjusted to spraying viscosity with acetone. The resultingcomposition could be sprayed without cob-webbing diificulties and thecoatings on primed and surfaced steel panels had high gloss as sprayed,good build, good scratch filling properties, and did not cause crazingwhen sprayed over a coating of acrylic lacquer. The weatheringproperties of the coatings were at least equal to those of coatings ofpolymers made in solution.

A corresponding polymer with a reduced viscosity at 0.5% solids at 25C., in ethylene dichloride 95 parts/ ethyl alcohol 5 parts of 0.35molecular weight equivalent 80,00090,000, was made in solution intoluene. In making a 35% solids solution, a mixture of toluene, acetone,fi-ethoxy ethyl acetate and methyl cyclohexanone had to be used toachieve the same viscosity as the solution of the dispersion polymer.When plasticized and thinned to spraying viscosity as described above itwas found that spraying aids had to be added to the solution to obtainsatisfactory application properties. Even then the coatings had a poorgloss and required much polishing, had poor scratch filling propertiesand caused crazing when used as a re-finish lacquer over existingacrylic lacquer.

Further, in the attempt to make the corresponding copolymer in solutionby using the same proportion of initiator/transfer agent as in thedispersion polymerization only 69.5% conversion was achieved in 5 hoursby which time the polymerization had ceased. (The dispersion processgave 95 conversion in 3-4 hours.) Furthermore, the acid value of thepolymer precipitated after 52.5% conversion was significantly higherthan at 69.5% con version showing that the distribution of themethacrylic acid was not even throughout the copolymer.

EXAMPLE 2 Methyl methacrylate homopolymer dispersions were made by theprocess described in Example 1. In this case, however, instead ofpreformed stabilizer, a stabilizer precursor was used, i.e. a solublepolymeric compound containing C=C groups which are copolymerized withpart of the main monomer to form the stabilizing graft copolymer. Thestabilizer precursor was a copolymer of lauryl methacrylate and glycidylmethacrylate (weight ratio 97:3 and molecular Weight about 50,000) theglycidyl groups of which were reacted with methacrylic acid to introduceon average 1.5 C=C groups per polymer molecule.

The charges were as follows:

(A) Parts 30% solution of precursor in petroleum ether (boiling point70-90" C.) 1.7 Petroleum ether (B.P. 70 C.) 320.0 10% solution ofp-octyl mercaptan in petroleum ether (B.P. 70-90 C.) 1.7Azodiisobutyronitrile 1.7 Methyl methacrylate 32.6

Methyl methacrylate 237.2 Azodiisobutyronitrile 0.5 p-Octyl mercaptansolution (as above) 8.75 Precursor solution (as above) 11.7

Methyl methacrylate 237.2 Azodiisobutyronitrile 0.5 p-Octyl mercaptansolution (as above) 8.75

Using the above proportion of mercaptan (0.38% of monomer), theviscosity average molecular weight of the disperse polymer was about70,000. On repeating the preparation using proportions of mercaptancorresponding to 0.76, 0.28, 0.19 and 0.095% of monomer, the molecularweight of the resulting polymers were about 45,000, 87,000, 101,000 and170,000 respectively. (Viscosity average molecular weights were obtainedfrom the reduced viscosity in toluene at 25 C. using formula of Chinaiet al., J. Poly. Science, 17, 391.)

The dispersions were dried by evaporation of the petroleum ether and thepolymer powder was dissolved in toluene 70 parts/ acetone 30 parts toform a solution containing 40% of polymer. Dicyclohexyl phthalate wasadded in a proportion of 30 parts per 70 parts polymer and rutiletitanium dioxide was dispersed in the polymer solution in a proportionof 0.5:1.0 by weight of pigment to polymer plus plasticizer.

When thinned to spraying viscosity with acetone all the compositionssprayed satisfactorily except that containing polymer of molecularweight 170,000 which showed signs of incipient cob-webbing. Even athigher average molecular weights the compositions had a higher solids ata given viscosity and sprayed wetter and gave higher build (filmthickness) than compositions based on polymers of average molecularweight about 70,000-80,000 (determined by method 445/61 of AmericanSociety for Testing Materials) made by solution polymerization. Thegloss of the coatings as sprayed was also better.

EXAMPLE 3 A series of dispersions of copolymers of composition methylmethacrylate 90 parts/B-ethoxy ethyl methacrylate 8 parts/methacrylicacid 2 parts, was made as described in Example 2. The average molecularweight of the disperse polymer was varied from about 45,000 to about170,000 ([1 ]=0.25 to 0.58 at 25 C. in ethylene dichloride 95 parts byvolume/ ethanol parts by volume) by varying the amount of p-octylmercaptan chain transfer agent from 0.75% to 0.1% by weight of the mixedmonomers. The polymers were directly transferred into toluene solutionas described in Example 1 to give solutions containing 40% polymer, thesolutions then being plasticized and pigmented as described in Example2. All the compositions sprayed satisfactorily except the one based onpolymer of molecular weight about 170,000 which showed incipientcob-webbing but gave a good film on the application of a second coat.Flow of the sprayed coatings was good and gloss of the dried coatingsvaried from 63-70%. The compositions did not cause crazing when appliedover a dried coat of acrylic lacquer under conditions where lacquersbased on the solutionprepared methyl methacrylate homo-polymer did causesevere crazing. The mechanical properties and adhesion of the sprayedcoatings over alkydzmelamine formaldehyde coatings which had beenrubbed-down with abrasive paper was good.

EXAMPLE 4 A dispersion of a copolymer of composition methyl methacrylate90 parts/fl-ethoxy ethyl methacrylate 8 parts/p,N:N-dimethyl aminoethylmethacrylate 2 parts was made and transferred into solution as describedin Example 1. The polymer had a viscosity average molecular weight ofabout 60,00070,000. As plasticizer was added a mixture of 70%dicyclohexyl phthalate and 30% butyl benzyl phthalate in a proportion of30 parts per 70 parts of polymer. Rutile titanium dioxide was dispersedin the solution in a proportion of 0521.0 by weight of pigment topolymer/plasticizer. When sprayed, the coatings had much improvedadhesion over rubbed-down stoved enamels (alkyd/melamine formaldehyde orurea formaldehyde finishes) both after air-drying and forcedryiug at 127C.

EXAMPLE 5 The dispersion polymerization of Example 1 was repeated using,in place of the graft copolymer stabilizer, 15.8 parts of a preformedstabilizer made by reacting poly-12-hydroxystearic acid (number averagemolecular weight about 1,500) with glycidyl methacrylate to give aterminal methacrylate group followed by copolymerization of thisunsaturated material with methyl methacrylate 98 parts/methacrylic acid2 parts at mass ratio of 1:1.

The hot dispersion was converted to solution by rapidly adding 704 partsof toluene heated to 95-100 C., to give a 43% solution on toluene,followed by distillation of petroleum ether and a small amount oftoluene until the solution contained about 45% by weight of polymer.Then acetone, 128 parts, was added to give a 40% solution.

EXAMPLE 6 The dispersion polymerization was carried out using 32 partsof petroleum ether (B.P. 7090 C.) heated to reflux to which was addedvia the returning cold distillate, one feed, containing 0.8 part of thestabilizer precursor, 100 parts methyl methacrylate, 0.5 part primaryoctyl mercaptan and 0.4 part azodiisobutyronitrile, over 2 hours, toproduce a dispersion containing 68% polymer. The coarse (5-6 microns)uniform particle size latex was converted into solution by adding 123parts toluene heated to 95-100 C., over the shortest time (2 minutes onlaboratory scale-1O minutes on workscale). Petroleum ether was thendistilled out with good agitation until the remaining solution contained45% polymer. This was then diluted to 40% solids with acetone. The finalsolvent composition was approximately 7% petroleum ether, 70% tolueneand 23% acetone.

What is claimed is:

1. A process for the manufacture of a solution in an organic liquid of apolymer of a member of the group consisting of amides of acrylic andmethacrylic acid and esters of acrylic and methacrylic acid withalcohols containing 1 to 8 carbon atoms,

said process comprising polymerizing a monomer selected from the groupconsisting of amides of acrylic acid and methacrylic acid and esters ofacrylic acid and methacrylic acid with alcohols containing 1 to 8 carbonatoms in an organic liquid dispersion medium in which said monomer issoluble and said polymer is insoluble, in the presence of an amount of astabilizer for the polymer particles produced by said process effectivefor the production of polymer particles having a particle size of atleast 0.5 micron, said stabilizer having a constituent which is solvatedby said organic liquid dispersion medium and another constituent ofdifferent polarity which associates with disperse particles of saidpolymer and thereby provides a sheath of solvated constituent aroundsaid particles, to form a dispersion of particles of said polymer,adding to said liquid dispersion medium a solvent for said polymer whichhas a boiling point higher than the organic liquid dispersion medium inwhich the monomer was polymerized, and evaporating said organic liquiddispersion medium until said polymer is dissolved.

2. A process as set forth in claim 1 in which the amount of saidstabilizer present during polymerization is 0.1 to 1% by weight of saiddispersed polymer.

3. A process as set forth in claim 1 in which, during thepolymerization, stabilizer is added concurrently with said monomer, theaddition of stabilizer being discontinued after 40 to 70% of the monomerhas been added.

4. A proces as set forth in claim 3 in which not less than of themonomer is added to the organic liquid during the polymerization.

5. A process as set forth in claim 4 in which the rate of addition ofmonomer is such that the proportion of free monomer in the reactionliquid is not more than 50% by weight.

6. A process as set forth in claim 1 in which the organic liquid presentduring polymerization contains a solvent for said polymer, theproportion of solvent being such that said polymer is at least insolublein said organic liquid.

7. A process as set forth in claim 6 in which the amount of the solventin said organic liquid present during polymerization is less than 40% byweight of said organic liquid.

8. A proces as set forth in claim 1 in which said organic liquid isevaporated by heating the dispersion to its refluxing temperature,heating the solvent for said polymer to at least as high a temperature,adding the heated solvent to the organic liquid and distilling theorganic liquid.

References Cited UNITED STATES PATENTS 3,095,388 6/1963 Osmond et a1.26034.2

10 3,218,287 11/1965 Schmidle et al 26024.2 3,317,635 5/1967 Osmond26034.2

FOREIGN PATENTS 606,479 1/ 1962 Belgium 260-342 US. Cl. X.R.

- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRE "MON Dated Patent NoInventofls) Desmondwilfrid John OSMOND et a1 It is certified that errorappears in the above-identified patent and that said Letters Pa tent arehereby corrected as shown below:

In the heading, insert the following information:

Assignee: Imperial Chemical Industries Limited,

London, England Signed and sealed this 17th day of September 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Cfficer COmI 'iSSILOIIGIof Patents

